It has been known in practice in the field of spin resonance spectroscopy to "edit" spectra where signals of different groups of nuclei heterodyne one with the other. By "editing" one understands different recording techniques which allow to filter out individual signals from the heterodyning spectra. Usually, this is effected by carrying out series of several measurements using different measuring parameters, and eliminating thereafter the undesirable signal contents by subtraction. Examples of such editing techniques for nuclear magnetic resonance applications have been described in the textbook entitled "Modern NMR Spectroscopy" by Sanders, Jeremy K. M. and Brian K. Hunter, Oxford University Press, 1987, pages 237 to 259. Other methods of this type are described by EP-OS 244 752 and EP-OS 166 559. In the case of these other known methods, uncoupled spins are suppressed by forming the difference between two measurements.
However, all of the techniques described above have the common disadvantage that for recording a single spectrum a plurality of measurements have to be performed successively in time, using different measuring parameters. While this presents no substantial problem to a laboratory in the case of durable chemical samples, considerable problems can result in cases where such nuclear magnetic resonance spectra are to be recorded on biological samples, i.e. on living tissue. This is true above all for in-vivo measurements to be carried out on patients where movement artefacts may lead to adulterations of the measured values.
In addition, subtracting measuring methods are connected with the fundamental drawback that the subtraction of high noise signal amplitudes may give rise to measuring errors which may be in the same range of magnitude as the useful signal.
According to other known methods, nuclear magnetic resonance spectra are recorded in a volume-selective way, i.e. only for a limited, geometrically defined area of a sample. This recording technique has gained particular importance in the fields of biological research and medicine. For, this recording technique enables, for example, a nuclear magnetic resonance spectrum to be recorded for a given, defined point in an inner organ of a patient. The technique of recording volume-selective nuclear magnetic resonance spectra has been known as such. Examples of this technique are found in the textbook entitled "Biomedical Magnetic Resonance Imaging" by Wehrili, Felix W., Derek Shaw and J. Bruce Kneeland, Verlag Chemie, 1988, pages 1 to 45 and 521 to 545.
Other known methods are, for example, the so-called SPARS method, which has been described by the U.S. publication "Journal of Magnetic Resonance", 67 (1986), page 148, and the so-called DIGGER method, described by the U.S. publication "Journal of Magnetic Resonance", 68 (1986), page 367. These known methods are volume-selective methods where the layers outside the selective volume area are saturated to leave only the selected volume area. However, it is the disadvantage of these two known methods, in particular of the DIGGER method, that they require a high r.f. power and that the pre-saturation r.f. pulses must be tuned very exactly in both methods as otherwise additional signals may be generated.
Another special method for volume-selective imaging of nuclear magnetic resonance spectra using three 90.degree. r.f. pulses spaced in time, while applying simultaneously different magnetic gradient field pulses in different coordinate directions, has been described for example by DE-OS 34 45 689. In the case of this known method, conventional stimulated spin echoes are produced.
Finally, it has been known from the U.S. publication "Magnetic Resonance in Medicine", 9 (1989), pages 254 to 260 to edit volume-selective spectra by means of homonuclear polarization transfer, using a single transfer. Now, when the nuclear magnetic resonance spectrum of a homonuclear or heteronuclear coupled spin system of the general form A.sub.n X.sub.m is to be recorded--this is of great interest in biomedical research as such measurements permit to draw conclusions regarding the metabolism in organic tissue--one frequently encounters the problem of overlapping signals. In the case of a homonuclear coupled spin system, both coupling partners consist of one and the same kind of nuclei, for example of protons (.sup.1 H), while in the case of heteronuclear coupled spin systems the coupling partners belong to different kinds of nuclei, for example the A group may be protons (.sup.1 H) while the X group may be a carbon isotope (.sup.13 C). If one regards lactate, a A.sub.3 X system, as a homonuclear example, the methyl group (CH.sub.3), has substantially the same chemical shift, i.e. line position in the spectrum, as the CH.sub.2 group of lipid, both chemical shifts being in the range of 1.35 ppm. Given the fact, however, that the lipid concentration may be considerably higher in living tissue, the CH.sub.2 signal of the lipid will mask the CH.sub.3 signal of the lactate. The same applies by analogy to any editing of the reonuclear A.sub.n X.sub.m systems, for example for an A.sub.3 X system such as methanol with a .sup.13 C enrichment.
Now, if in the case of the first mentioned example a volume-selective lactate measurement were carried out in a lipid environment, using the known editing techniques, where, as has been mentioned before, two measurements have to be performed successively in time using different parameters, problems would be encountered if the patient should move during the two measurements. For, any such movement would give rise to artefacts, which would influence the measurements differently so that it would be necessary, during the subsequent substracting process, to work out, by suitable editing, not only the desired isolated CH.sub.3 signals of the lactate, but also the undesired lipid artefacts.
Although the invention will be explained for the purposes of the present invention by way of an application chosen from the nuclear magnetic resonance (NMR) field, it is understood that it can be applied also in connection with other forms of spin resonance, in particular electron paramagnetic resonance (EPR) or nuclear/electron double resonance techniques (ENDOR, ELDOR, NEDOR, Overhauser, etc.).
Further, although the invention will be described using the simple example of scalar coupling (J) it is understood that it is suited also for application in connection with other coupling types, for example, dipole coupling.
It has further been known, in selective editing of metabolite signals, to make use of multiple-quantum transitions (MQ) or zero-quantum transitions (ZQ). Examples of this method are found in the U.S. publication "Journal of Magnetic Resonance", 64 (1985), page 38, in U.S. publication "Journal of Magnetic Resonance", 78 (1988), page 355, in U.S. publication "Journal of Magnetic Resonance", 77 (1988), page 382, and in U.S. publication "Magnetic Resonance in Medicine", 9 (1989), page 32.
The essential drawback of all subtracting methods lies in their dependence on the precision of the subtraction process. This is a critical factor in all in-vivo measurements where dominating resonances of water and lipid may be encountered. Another drawback lies in the fact that two-dimensional measurements must be performed in order to extract the metabolite signal. A known one-dimensional method made use of selective excitation and pulsed field gradients for selecting exclusively zero-quantum coherences so that it was possible, with a single measurement, to obtain satisfactory spectral editing and to suppress solvents. However, these methods are connected with the essential drawback that losses in the signal-to-noise ratio are encountered. This is so, because in any case zero-quantum and multiple-quantum coherences will always be excited jointly, but only one of these coherences will be converted to an observable single-quantum signal (SQ) while the other will be dephased by a field gradient or eliminated by phase shifting. If zero-quantum coherences are selected, only one fourth at the maximum of the coherence will be available for subsequent application of field gradients, and the effectivity of the formation of multiple-quantum coherences, if desired, may depend on a plurality of spin-spin couplings, as for example in the case of ethanol where single-quantum and triple-quantum contents (SQ, TQ) get lost. It has also been proposed to make use of selective polarization transfer methods, but these, too, lead to signal losses as a result of the transfer direction, or as a result of the dependence of the polarization transfer on the r.f. pulse angle, so that extensive cyclical phase shifting is required. For more details regarding the last-mentioned effects, reference is made to the U.S. publication "Journal of Magnetic Resonance", 66 (1986), page 86, and to the U.S. publication "Progress in NMR Spectroscopy", 16 (1983), page 163.
U.S. Pat. No. 4,521,732 describes a further method where a series of successive r.f. pulses is directed upon a sample comprising at least two different types of nuclie, wherein the first and the third pulses act selectively on the one kind of nuclei while the second pulse acts selectively on the other kind of nuclei, and wherein the time interval between each pair of successive pulses is such as to permit the development of a heteronuclear scalar coupling interaction.
Finally, still other pulse sequences used for examining heteronuclear systems have been known from U.S. Pat. No. 4,680,546, U.S. Pat. No. 4,238,735 and U.S. Pat. No. 4,703,270.